Liquid fuel burner having reduced thermal stress and rapid start-up time

ABSTRACT

A burner for burning gasified liquid fuel such as oil, in which the fuel is gasified by being mixed with hot burned gases drawn from the combustion chamber, is provided in which the normal thermal stresses found in such burners due to the return of the hot burned gases for gasifying are substantially reduced and, at the same time, start-up time is reduced. Minimization of the thermal stresses is accomplished by utilizing the heat of vaporization of the oil or other fuel to cool the hot returning gases, and so limit thermal expansion, by having the gasification of the fuel commence close to the hot gas outlet of the burner, rather than at a more remote distance, such as a gasification chamber. This permits elimination of the gasification chamber, which normally has been required to provide a preheating area for the oil, and so further reduces stresses. With such elimination, rapid start-up is provided by preliminarily burning ungasified fuel introduced by the air and fuel system of the principal unit.

Unite States Patent Reichhelm et a1.

[ 5] Dec. 12, 1972 [54] LIQUID FUEL BURNER HAVING REDUCED THERMAL STRESSAND RAPID START-UP TIME [72] Inventors: Robert Reichhelm, Lancaster; Al-

bert Morrison, Jr., Leola, both of [73] Assignee: Burnham Corporation,Irvingtonon-Hudson, N.Y.

[22] Filed: Jan. 11, 1971 [21] Appl. No.: 105,212

[52] US. Cl ..431/l16 [51] Int. Cl ..F23m 9/06 [58] Field ofSearch..43l/115, 116,215

[56] References Cited UNITED STATES PATENTS 3,361,183 1/1968 Reichhelm..431/1 16 2,688,360 9/1954 Haynes et al.. ..43l/1l5 1,492,674 5/1924Chapman ..43l/115 Primary Examiner-Edward G. Favors Attorney-Bryan,Parmelee, Johnson & Bollinger [57 ABSTRACT A burner for burning gasifiedliquid fuel such as oil, in which the fuel is gasified by being mixedwith hot burned gases drawn from the combustion chamber, is provided inwhich the normal thermal stresses found in such burners due to thereturn of the hot burned gases for gasifying are substantially reducedand, at the same time, start-up time is reduced. Minimization of thethermal stresses is accomplished by utilizing the heat of vaporizationof the oil or other fuel to cool the hot returning gases, and so limitthermal expansion, by

I having the gasification of the fuel commence close to the hot gasoutlet of the burner, rather than at a more remote distance, such as agasification chamber. This permits elimination of the gasificationchamber, which normally has been required to provide a preheating areafor the oil, and so further reduces stresses. With such elimination,rapid start-up is provided by preliminarily burning ungasified fuelintroduced by the air and fuel system of the principal unit.

14 Claims, 3 Drawing Figures PATENTED DEC 12 I972 FIGI INVENTORS. ROBERTREICHHELM BY ALBERT MORRISON JR.

FIG. 3

TTORNEYS LIQUID FUEL BURNER HAVING REDUCED THERMAL STRESS AND RAPIDSTART-UP TIME BACKGROUND OF THE INVENTION This invention relates to aburner in which a liquid fuel, such as oil, is first gasified, mixedwith air, and then burned. Gasification occurs by mixing the fuel withhot burned gases coming from the burner chamber.

Burners using ga'sified fuel are not new, although the techniques forgasification, and the success of the techniques, has varied.

Gasification systems are shown in various of the patents of the presentinventor, Reichhelm, such as U.S. Pat. Nos. 2,197,347, and 3,308,868where a portion of the oil is burned and used to gasify the remainder;U.S. Pat. Nos. 2,735,481 and 3,320,743 where the fuel is gasified bybeing mixed with heated air; and U.S. Pat. No. 3,361,183, wheregasification is obtained by recirculating a portion of the combustiongases into admixture with the fuel and a Venturi is used to draw in thegasified fuel and the hot combustion gases. In addition, multipleburners, such as those adapted to burn different fuels, or to burn thesame fuel in different ways, have existed. Examples of these areReichhelm U.S. Pat. No. 3,308,868 and French patent In burners beingused for gasifying fuel by mixing oi with hot products of combustion,such as that shown in Reichhelm U.S. PatpNo. 3,361,183 or in Reichhelmcopending application Ser. No. 56,482, filed July 20, 1970, thegasification of the fuel is shown taking place in a separate chamber.This chamber has hot products of combustion entering the chamber at oneinlet, liquid fuel entering it at another, and a mixture ofthe two beingdrawn out at an outlet port after gasification; It then goes into amixing chamber for mixing with compressed air and into a burning chamberfor burning. This structure, having a gasification chamber, can preheatfor starting for the use of an electric heater around the gasificationchamber. The structure, however, also results in undesirable thermalstresses due to the fact that the hot combustion gases must pass fromthe burning chamber to the gasification chamber. This method of preheatalso takes a relatively long start-up time.

SUMMARY OF THE INVENTION The inherent heat of vaporization of agasifiable fuel, such as oil is used to reduce thermal stresses within agasification type burner by reducing the temperature of the componentsmost subject to stress. This is accomplished by injecting the liquidfuel into the hot combustion gas stream at an earlier point in thesystems operation so that its heat of vaporization will be taken upsooner and so serve to reduce the temperature of the returned gasesbefore thermal expansion occurs. This permits elimination of thegasification chamber, further reducing thermal stresses and also addingto compaction.

The fuel and air supply system used for the above is also integrallyused to provide a start-up system for a cold burner using the fuelwithout gasification.

DESCRIPTION OF THE DRAWINGS The invention is best understood throughreference to the drawings, as follows:

FIG. 1 is a schematic drawing showing flow and operation of the unit.

FIG. 2 is a perspective view of the burner.

FIG. 3 is a side elevation of the burner partially cut away in a sectionto show portions of the burning chamber and return gas duct.

DETAILED DESCRIPTION OF THE INVENTION Structure port 14. Port 14 leadsfrom chamber rearwardly of the unit in the general direction of burnerchamber inlet or throat 11, through the refractory material to aninterconnecting point 15 where it leads to interconnecting means or duct17, normally a metal pipe. The purpose of outlet 14 is to carry hot,burned gases, i.e., the products of combustion, from the end of burnerchamber 10 back towards the mixing chamber 6, the gases being used togasify or vaporize liquid fuel.

Mixing chamber 6 mixes the gasified fuel with combustion air and feedsit into the burning chamber 10. The air, under pressure, enters mixingchamber 6 through air inlet duct 20 and air inlet 7. Duct 20, as itapproaches mixing chamber 6 is constructed to form a Venturi 22 withinchamber 6 to create a suction in line 17. This suction serves to drawthe hot burned gases from burning chamber 10 through outlet 14 intochamber 6.

The liquid fuel to be burned comes from a fuel source inlet 25. Source25 branches to form fuel supply system 7. Branch 26 passes throughmetering valve 27 to an inlet 28 in interconnecting duct 17. Fuel foruse in the start-up system, to be described below, follows duct 30through valve 31 and duct 32 to a nozzle 33 located just within theVenturi construction 22 in air inlet 20.

It should be noted that the oil inlet 28 into the interconnecting duct17 is positioned proximate to the exit from the refractory material 5near point 15. Preferably, it should be located as close to therefractory material as is practical. The oil may be fed by eithermechanical or gravity pressure, as is desired. The oil is mixed with thehot burned gas in interconnecting duct 17 at oil inlet 28. Duct 17 thencarries the mixture of oil and hot combustion gases to mixing chamber 6;the mixture enters chamber 6 at burned gas and fuel inlet Anelectrically operated igniter plug 35 is located at the burning chamberinlet 11 or slightly toward the mixing chamber from that point. It hasbeen found that by having the igniter 35 positioned slightly outside of,but before, the burning chamber, the plug has less tendency to becomefouled by products of combustion.

It will be noted that the above structure permits elimination of thegasification chamber, (compare chamber 14 in Reichhelm U.S. Pat. No.3,361,183). It will also be noted that the oil has been mixed with hotgas at a point much closer to the point of exit of hot gases from therefractory material (point 15; compare the liquid fuel inlet 21, FIG. 1,of the Reichhelm patent). If the gasification chamber is eliminated,however, the electric preheater is also eliminated (compare heater 18 ofReichhelm U.S. Pat. No. 3,361,183); in its stead a completely distinctbut integrally operating start-up unit identified by circuit 30 has beenprovided. This start-up unit is operatively associated with fuel supplysystem 7.

This start-up system is integral with the overall system using and thesame fuel source. It is adapted for sequential operation, i.e.,operation of the start-up followed by operation of the principalgasifying system. The start-up unit has liquid fuel passing from source25 through duct 30, valve 31, and duct 32 to nozzle 33 within, orproximate to, Venturi 22.

Operation Operation of the liquid fuel burner will be first described inthe operating stage, and then the start-up aspects considered.

In operation, oil or other liquid fuel capable of being gasified entersthe system from source 25 passes through duct 26 and valve 27 and is fedinto the hot combustion gases at the intersection point of duct 28 andinterconnecting duct 17. Meanwhile hot gases, a portion of the totalvolume of combustion gases, have been taken from combustion chamberthrough hot gas discharge port 14 passing through refractory block 5 topoint 15 and then into interconnecting duct 17 and into mixing chamber6. Air under pressure enters through air inlet 20 and passes throughVenturi 22 into mixing chamber 6. The passage of the air through Venturi22 creates a suction which draws the hot burned combustion gases throughinlet 14 and duct 17 into mixing chamber 6. These hot combustion gases(mixed with and carrying gasified oil) are mixed with the air in chamber6 and pass through burning chamber inlet 11 to be burned in chamber 10.Electrical plug 35 pro vides ignition. Most of the combustion gases arethen discharged through discharge opening 12 with only a small portionreturning through outlet 14.

As noted above, the oil is injected into interconnecting duct 17 asclose to the source of hot gas as is practical. Since the oil, as withany vaporizable liquid, requires heat of vaporization, the heat energynecessary for the gasification of the liquid fuel comes from the hotburned gases from duct 14. Thus the gasification of the oil serves tocool the hot gases. By placing inlet 28 close to the source of hot gas,the cooling takes place early in interconnecting pipe 17. Thus pipe 17,a gas delivery pipe, does not reach the usual red-hot temperaturenormally found in prior art structures. Reduction of the temperature ofthe duct 17 reduces its thermal expansion (the expansion primarily beingin a longitudinal direction), and so reduces thermal stresses in thearea between mixing chamber 6 and refractory material 5. This isparticularly important at points of junction.

Interconnecting duct 17 must have sufficient length after injection ofthe oil to allow adequate time for gasification of the fuel, if agasification chamber is not used. Determination of adequate lengthrelative to quantities of returned gas used can readily be made byobserving the color of the flame in burner chamber 10. If blue, adequategasification has taken place; if yellow is present, it is likely thatsome of the fuel has remained liquid;

If duct 17 has sufficient length after inlet 28, relative to the volumeand temperature of hot return gases in duct 17, substantially completegasification will take place in duct 17 prior to the oil reaching mixingchamber 6. Under these circumstances and if there remains sufficientheat energy in the mixture entering inlet 13, additional liquid fuel mayalso be injected at nozzle 33 for enhanced heat capacity of the overallburner. This, then, would result in two-stage operation if all oil isgasified, but will have yellow in it if there is too much oil fromnozzle 33 (acceptable under some circumstances).

Though gasification using hot, burned gases only appears to bepreferable, the presence of air with the hot gas in duct 17 is notobjectionable provided it does not cool the gas to the point ofincomplete gasification of fuel (i.e., yellow flame) and provided thatpremature ignition does not occur.

When the burner unit is cold and needs to be started, feedback ofcombustion gases will not, of course, serve to gasify the oil being fedinto the system. Consequently, some other method of preheating isnecessary. In the present invention the start-up system is an integralpart of the total system since it uses the same air supply, Venturi, andfuel supply. The oil passes through ducts 30 and 32 through the nozzle33 and is picked up by the air entering through the air inlet duct 20.It will be ignited by plug 35 and will burn in burning chamber 10. Thisstart-up operation is essentially full operation of the burner, not justa pilot light, but uses ungasified fuel and so is less efficient.

Valves 27 and 31 are used to determine which oil supply system is used.At start-up, valve 31 is open and valve 27 closed, allowing the oil toflow to nozzle 33. Once the warm-up period is over, valve 27 is openedto permit oil to flow into interconnecting duct 17, and valve 31 isclosed. If desired, valves 27 and 31 may be combined at inlet source 25and may also be operated automatically by a sequence timing control. If,after start-up, two-stage gasification is to be used, both valves beleft open by an amount that will give desired combustion balance.

It has been found that a unit can be started in under half a minute tothe point where the hot burned gases can vaporize the oil. One unitbuilt has had a preheating time of approximately 16 seconds.

What is claimed is:

1. A liquid fuel burner for burning a gasifiable liquid fuel such as oiland having reduced thermal stresses, including a burner made ofrefractory material and having a fire chamber, said chamber having afuel mixture inlet port, a burned gas removal port, and a hot burned gasfeedback port,

a mixing chamber for mixing said fuel and air, said mixing chamber beingconnected to said inlet port,

a burned gas and fuel inlet to said mixing chamber,

means directly interconnecting said burned gas removal port to saidburned gas and fuel inlet,

an air inlet to said mixing chamber for directing air under pressureinto said mixing chamber, said air inlet including a Venturi for drawingburned gas and fuel into said chamber through said interconnecting meansand said burned gas and fuel inlet, and

a gasifiable liquid fuel inlet connected to said interconnecting meansproximate to said refractory material,

whereby said fuel will be gasified by hot burned gas as it passesthrough said interconnecting means and the heat of. vaporization of saidfuel will cool said hot burned gases, thereby reducing the temperaturesthereof and reducing thermal stresses therein.

2. A liquid fuel burner as set forth in claim 1 in which there isinsufficient air present in said interconnecting means to supportcombustion of said gasified fuel prior to its reaching said fuel mixtureinlet port.

3. A liquid fuel burner as set forth in claim 1 including ignition meanspositioned proximate to, but before, said fuel mixture inlet port, forigniting said mixture of said fuel and air.

4. In a liquid fuel burner for burning a gasifiable liquid fuel such asoil which includes a burner made of refractory material and having afire chamber therein, said fire chamber having a fuel mixture inletport, a burned gas removal port and a hot burned gas feedback port, asource of air under pressure, a mixing chamber for mixing said fuel andair and being connected to said inlet port, that improvement forreducing thermal stresses in hot gas feedback including means directlyinterconnecting said hot burned gas feedback port with saidmixingchamber,

means for introducing said gasifiable liquid fuel into saidinterconnecting means, so proximate to said refractory material thatsaid fuel will be gasified by said hot burned gas as it passes throughsaid interconnecting means and the heat of vaporization of said fuelwill cool said hot burned gases, thereby reducing the temperaturesthereof and reducing thermal stresses.

5. A liquid fuel burner improvement as set forth in claim 4 including ametering valve associated with said gasifiable liquid fuel inlet tocontrol the rate of flow into said inlet.

6. In a liquid fuel burner for burning a gasifiable liquid fuel such asoil which includes a burner made of refractory material and having afire chamber therein, said fire chamber having a fuel mixture inletport, a burned gas removal port and a hot burned gas feedback port,means for connecting said hot burned gas feedback port to said fuelmixture inlet port, a source of air under pressure, a mixing chamber formixing said fuel and air and being connected to said inlet port, and

means for introducing liquid fuel into said connecting means so that itmay be gasified prior to combustion, that improvement for start-up ofsaid burner including:

a liquid fuel nozzle disposed in said air source,

ducts interconnecting said nozzle with said liquid fuel introducingmeans, and

control valve means for controlling flow of said liquid fuel to saidnozzle or to said connecting means,

whereby said burner may be started by use of said nozzle to injectliquid fuel into said air for burning in said fire chamber prior to useof gasified fuel.

7. A liquid fuel burner improvement as set forth in claim 6 including asequence timing control for said control valve means.

8. A liquid fuel burner for burning a gasifiable liquid fuel such as oiland having reduced thermal stresses, including a burner made ofrefractory material and having a fire chamber, said chamber having afuel mixture inlet port, a burned gas removal port, and a hot burned gasfeedback port,

a mixing chamber for mixing said fuel and air, said mixing chamber beingconnected to said inlet port,

a burned gas and fuel inlet to said mixing chamber,

means interconnecting said burned gas removal port to said burned gasand fuel inlet,

said interconnecting means being a metallic pipe running between saidburned gas feedback port and said burned gas and fuel inlet,

an air inlet to said mixing chamber for directing air under pressureinto said mixing chamber, said air inlet including a Venturi for drawingburned gas and fuel into said chamber through said interconnecting meansand said burned gas and fuel inlet, and

a gasifiable liquid fuel inlet connected to said interconnecting meansproximate to said refractory material,

whereby said fuel will be gasified by hot burned gas as it passesthrough said interconnecting means and the heat of vaporization of saidfuel will cool said hot burned gases, thereby reducing the temperaturesthereof and reducing thermal stresses therein.

9. A liquid fuel burner as set forth in claim 8 in which said gasifiableliquid fuel inlet is sufficiently proximate to said refractory materialrelative to the length of said interconnecting pipe and the temperatureof said burned gas so as to gasify said fuel as it passes through saidinterconnecting means sufficiently to substantially avoid appearance ofyellow in the flame in said fire chamber.

10. A liquid fuel burner for burning a gasifiable liquid fuel such asoil and having reduced thermal stresses, including a burner made ofrefractory material and having a fire chamber, said chamber having afuel mixture inlet port, a burned gas removal port, and a hot burned gasfeedback port,

a mixing chamber for mixing said fuel and air, said mixing chamber beingconnected to said inlet port,

a burned gas and fuel inlet to said mixing chamber,

means interconnecting said burned gas removal port to said burned gasand fuel inlet,

an air inlet to said mixing chamber for directing air under pressureinto said mixing chamber, said air inlet including a Venturi for drawingburned gas and fuel into said chamber through said interconnecting meansand said burned gas and fuel inlet,

a gasifiable liquid fuel inlet connected to said interconnecting meansproximate to said refractory material,

and a starter unit for creating a tire in said fire chamber whenstarting said burner so as to produce hot burned gas for feedback andgasification, said starter unit including a nozzle for inserting saidfuel in its liquid form into said air inlet,

whereby said fuel will be gasified by hot burned gas as it passesthrough said interconnecting means and the heat of vaporization of saidfuel will cool said hot burned gases, thereby reducing the temperaturesthereof and reducing thermal stresses therein.

11. A liquid fuel burner as set forth in claim 10 including fuel supplyfor said fuel inlet and for said nozzle, and control valve means forcontrolling flow of said fuel to said nozzle or said fuel inletsequentially.

12. A liquid fuel burner for burning a gasifiable liquid fuel such asoil and having reduced thermal stresses, including a burner made ofrefractory material and having a fire chamber, said chamber having afuel mixture inlet port, a burned gas removal port, and a hot burned gasfeedback port,

a mixing chamber for mixing said fuel and air, said mixing chamber beingconnected to said inlet port,

a burned gas and fuel inlet to said mixing chamber,

means interconnecting said burned gas removal port to said burned gasand fuel inlet,

an air inlet to said mixing chamber for directing air under pressureinto said mixing chamber, said air inlet including a Venturi for drawingburned gas and fuel into said chamber through said interconnecting meansand said burned gas and fuel inlet,

a gasifiable liquid fuel inlet connected to said interconnecting meansproximate to said refractory material,

and means for inserting additional liquid fuel into said mixing chamberat said air inlet to provide a second stage of liquid fuel gasification,said second stage utilizing heat energy remaining in said hot burned gasentering said mixing chamber,

whereby said fuel will be gasified by hot burned gas as it passesthrough said interconnecting means and the heat of vaporization of saidfuel will cool said hot burned gases, thereby reducing the temperaturesthereof and reducing thermal stresses therein.

13. In a liquid fuel burner for burning a gasifiable liquid fuel such asoil which includes a burner made of refractory material and having afire chamber therein,

said fire chamber having a fuel mixture inlet port, a burned gas removalport and a hot burned gas feedback port, a source of air under pressure,a mixing chamber for mixing said fuel and air and being connected tosaid inlet port,

that improvement for reducing thermal stresses in hot gas feedbackincluding means interconnecting said hot burned gas feedback port withsaid mixing chamber, means for introducing said gasifiable liquid fuelinto said interconnecting means, so proximate to said refractorymaterial that said fuel will be gasified by said hot burned gas as itpasses through said interconnecting means and the heat of vaporizationof said fuel will cool said hot burned gases, thereby reducing thetemperatures thereof and reducing thermal stresses, and a starter unitto provide initial hot burned gas for feedback, said starter unitincluding means for inserting ungasified fuel into the air stream insaid mixing chamber. 14. In a liquid fuel burner for burning agasifiable liquid fuel which. includes a fire chamber and a mixinchamber for mixing air with said liquid fuel after gasi lcation and forfeeding same to said fire chamber, that improvement including duct meansinterconnecting said fire chamber and mixing chamber to carry hotcombustion gases from said fire chamber to said mixing chamber, saidduct means interconnecting said burner and said mixing chamber bysubstantially the most direct path, said duct means having a fuel inletarea for receiving said liquid fuel prior to gasification, and said fuelinlet area being positioned sufficiently close to said firing chamberand sufficiently far from said mixing chamber as to permit substantialgasification of said fuel in said duct means before said fuel as carriedby said hot combustion gases reaches said mixing chamber and as topermit the heat of vaporization of said oil to reduce the temperature ofsaid duct means.

1. A liquid fuel burner for burning a gasifiable liquid fuel such as oiland having reduced thermal stresses, including a burner made ofrefractory material and having a fire chamber, said chamber having afuel mixture inlet port, a burned gas removal port, and a hot burned gasfeedback port, a mixing chamber for mixing said fuel and air, saidmixing chamber being connected to said inlet port, a burned gas and fuelinlet to said mixing chamber, means directly interconnecting said burnedgas removal port to said burned gas and fuel inlet, an air inlet to saidmixing chamber for directing air under pressure into said mixingchamber, said air inlet including a Venturi for drawing burned gas andfuel into said chamber through said interconnecting means and saidburned gas and fuel inlet, and a gasifiable liquid fuel inlet connectedto said interconnecting means proximate to said refractory material,whereby said fuel will be gasified by hot burned gas as it passesthrough said interconnecting means and the heat of vaporization of saidfuel will cool said hot burned gases, thereby reducing the temperaturesthereof and reducing thermal stresses therein.
 2. A liquid fuel burneras set forth in claim 1 in which there is insufficient air present insaid interconnecting means to support combustion of said gasified fuelprior to its reaching said fuel mixture inlet port.
 3. A liquid fuelburner as set forth in claim 1 including ignition means positionedproximate to, but before, said fuel mixture inlet port, for ignitingsaid mixture of said fuel and air.
 4. In a liquid fuel burner forburning a gasifiable liquid fuel such as oil which includes a burnermade of refractory material and having a fire chamber therein, said firechamber having a fuel mixture inlet port, a burned gas removal port anda hot burned gas feedback port, a source of air under pressure, a mixingchamber for mixing said fuel and air and being connected to said inletport, that improvement for reducing thermal stresses in hot gas feedbackincluding means directly interconnecting said hot burned gas feedbackport with said mixing chamber, means for introducing said gasifiableliquid fuel into said interconnectiNg means, so proximate to saidrefractory material that said fuel will be gasified by said hot burnedgas as it passes through said interconnecting means and the heat ofvaporization of said fuel will cool said hot burned gases, therebyreducing the temperatures thereof and reducing thermal stresses.
 5. Aliquid fuel burner improvement as set forth in claim 4 including ametering valve associated with said gasifiable liquid fuel inlet tocontrol the rate of flow into said inlet.
 6. In a liquid fuel burner forburning a gasifiable liquid fuel such as oil which includes a burnermade of refractory material and having a fire chamber therein, said firechamber having a fuel mixture inlet port, a burned gas removal port anda hot burned gas feedback port, means for connecting said hot burned gasfeedback port to said fuel mixture inlet port, a source of air underpressure, a mixing chamber for mixing said fuel and air and beingconnected to said inlet port, and means for introducing liquid fuel intosaid connecting means so that it may be gasified prior to combustion,that improvement for start-up of said burner including: a liquid fuelnozzle disposed in said air source, ducts interconnecting said nozzlewith said liquid fuel introducing means, and control valve means forcontrolling flow of said liquid fuel to said nozzle or to saidconnecting means, whereby said burner may be started by use of saidnozzle to inject liquid fuel into said air for burning in said firechamber prior to use of gasified fuel.
 7. A liquid fuel burnerimprovement as set forth in claim 6 including a sequence timing controlfor said control valve means.
 8. A liquid fuel burner for burning agasifiable liquid fuel such as oil and having reduced thermal stresses,including a burner made of refractory material and having a firechamber, said chamber having a fuel mixture inlet port, a burned gasremoval port, and a hot burned gas feedback port, a mixing chamber formixing said fuel and air, said mixing chamber being connected to saidinlet port, a burned gas and fuel inlet to said mixing chamber, meansinterconnecting said burned gas removal port to said burned gas and fuelinlet, said interconnecting means being a metallic pipe running betweensaid burned gas feedback port and said burned gas and fuel inlet, an airinlet to said mixing chamber for directing air under pressure into saidmixing chamber, said air inlet including a Venturi for drawing burnedgas and fuel into said chamber through said interconnecting means andsaid burned gas and fuel inlet, and a gasifiable liquid fuel inletconnected to said interconnecting means proximate to said refractorymaterial, whereby said fuel will be gasified by hot burned gas as itpasses through said interconnecting means and the heat of vaporizationof said fuel will cool said hot burned gases, thereby reducing thetemperatures thereof and reducing thermal stresses therein.
 9. A liquidfuel burner as set forth in claim 8 in which said gasifiable liquid fuelinlet is sufficiently proximate to said refractory material relative tothe length of said interconnecting pipe and the temperature of saidburned gas so as to gasify said fuel as it passes through saidinterconnecting means sufficiently to substantially avoid appearance ofyellow in the flame in said fire chamber.
 10. A liquid fuel burner forburning a gasifiable liquid fuel such as oil and having reduced thermalstresses, including a burner made of refractory material and having afire chamber, said chamber having a fuel mixture inlet port, a burnedgas removal port, and a hot burned gas feedback port, a mixing chamberfor mixing said fuel and air, said mixing chamber being connected tosaid inlet port, a burned gas and fuel inlet to said mixing chamber,means interconnecting said burned gas removal port to said burned gasand fuel inlet, an air inlet to said mixing chamber for directing aiRunder pressure into said mixing chamber, said air inlet including aVenturi for drawing burned gas and fuel into said chamber through saidinterconnecting means and said burned gas and fuel inlet, a gasifiableliquid fuel inlet connected to said interconnecting means proximate tosaid refractory material, and a starter unit for creating a fire in saidfire chamber when starting said burner so as to produce hot burned gasfor feedback and gasification, said starter unit including a nozzle forinserting said fuel in its liquid form into said air inlet, whereby saidfuel will be gasified by hot burned gas as it passes through saidinterconnecting means and the heat of vaporization of said fuel willcool said hot burned gases, thereby reducing the temperatures thereofand reducing thermal stresses therein.
 11. A liquid fuel burner as setforth in claim 10 including fuel supply for said fuel inlet and for saidnozzle, and control valve means for controlling flow of said fuel tosaid nozzle or said fuel inlet sequentially.
 12. A liquid fuel burnerfor burning a gasifiable liquid fuel such as oil and having reducedthermal stresses, including a burner made of refractory material andhaving a fire chamber, said chamber having a fuel mixture inlet port, aburned gas removal port, and a hot burned gas feedback port, a mixingchamber for mixing said fuel and air, said mixing chamber beingconnected to said inlet port, a burned gas and fuel inlet to said mixingchamber, means interconnecting said burned gas removal port to saidburned gas and fuel inlet, an air inlet to said mixing chamber fordirecting air under pressure into said mixing chamber, said air inletincluding a Venturi for drawing burned gas and fuel into said chamberthrough said interconnecting means and said burned gas and fuel inlet, agasifiable liquid fuel inlet connected to said interconnecting meansproximate to said refractory material, and means for insertingadditional liquid fuel into said mixing chamber at said air inlet toprovide a second stage of liquid fuel gasification, said second stageutilizing heat energy remaining in said hot burned gas entering saidmixing chamber, whereby said fuel will be gasified by hot burned gas asit passes through said interconnecting means and the heat ofvaporization of said fuel will cool said hot burned gases, therebyreducing the temperatures thereof and reducing thermal stresses therein.13. In a liquid fuel burner for burning a gasifiable liquid fuel such asoil which includes a burner made of refractory material and having afire chamber therein, said fire chamber having a fuel mixture inletport, a burned gas removal port and a hot burned gas feedback port, asource of air under pressure, a mixing chamber for mixing said fuel andair and being connected to said inlet port, that improvement forreducing thermal stresses in hot gas feedback including meansinterconnecting said hot burned gas feedback port with said mixingchamber, means for introducing said gasifiable liquid fuel into saidinterconnecting means, so proximate to said refractory material thatsaid fuel will be gasified by said hot burned gas as it passes throughsaid interconnecting means and the heat of vaporization of said fuelwill cool said hot burned gases, thereby reducing the temperaturesthereof and reducing thermal stresses, and a starter unit to provideinitial hot burned gas for feedback, said starter unit including meansfor inserting ungasified fuel into the air stream in said mixingchamber.
 14. In a liquid fuel burner for burning a gasifiable liquidfuel which includes a fire chamber and a mixing chamber for mixing airwith said liquid fuel after gasification and for feeding same to saidfire chamber, that improvement including duct means interconnecting saidfire chamber and mixing chamber to carry hot combustion gases from saidfire chamber to said mixing chamber, said duct means interconnectIngsaid burner and said mixing chamber by substantially the most directpath, said duct means having a fuel inlet area for receiving said liquidfuel prior to gasification, and said fuel inlet area being positionedsufficiently close to said firing chamber and sufficiently far from saidmixing chamber as to permit substantial gasification of said fuel insaid duct means before said fuel as carried by said hot combustion gasesreaches said mixing chamber and as to permit the heat of vaporization ofsaid oil to reduce the temperature of said duct means.